zxld1352 Zetex Semiconductors plc., zxld1352 Datasheet - Page 5

• Manufacturer Part Number: zxld1352
• Description: 350ma Led Driver With Internal Switch And Enhanced Pwm Dimming Range
• Manufacturer: Zetex Semiconductors plc.

Device description
The device, in conjunction with the coil (L1) and current sense resistor (R
continuous-mode buck converter.
Device operation
Operation can be best understood by assuming that the ADJ pin of the device is unconnected and
the voltage on this pin (V
When input voltage V
output from the current sense circuit. Under this condition, the (-) input to the comparator is at
ground and its output is high. This turns MN on and switches the LX pin low, causing current to
flow from V
and L1 to produce a voltage ramp (V
forced across internal resistor R1 by the current sense circuit and produces a proportional current
in internal resistors R2 and R3. This produces a ground referred rising voltage at the (-) input of
the comparator. When this reaches the threshold voltage (V
low and MN turns off. The comparator output also drives another NMOS switch, which bypasses
internal resistor R3 to provide a controlled amount of hysteresis. The hysteresis is set by R3 to be
nominally 15% of V
When MN is off, the current in L1 continues to flow via D1 and the LED(s) back to V
decays at a rate determined by the LED and diode forward voltages to produce a falling voltage
at the input of the comparator. When this voltage returns to V
high again. This cycle of events repeats, with the comparator input ramping between limits of
V
Switching thresholds
With V
100mV (measured on the I
then defined by this voltage and Rs according to:
I
Nominal ripple current is ±15mV/R
Adjusting output current
The device contains a low pass filter for noise suppression between the ADJ pin and the threshold
comparator and an internal current limiting resistor (50k nom) between ADJ and the internal
reference voltage. This allows the ADJ pin to be overdriven with either DC or PWM signals to
adjust the output current. The filter is first order, comprising one section with a cut-off frequency
of nominally 600kHz.
Details of the different modes of adjusting output current are given in the applications section.
Output shutdown
The ADJ pin drives the shutdown circuit. When the input voltage to this circuit falls below the
threshold (0.2V nom), the internal regulator and the output switch are turned off. The voltage
reference remains powered during shutdown to provide the bias current for the shutdown circuit.
Quiescent supply current during shutdown is nominally 20 A and switch leakage is below 1 A.
Issue 1 - August 2008
© Diodes Incorporated 2008
OUTnom
ADJ
± 15%.
ADJ
=100mV/R
=V
IN
REF
to ground, via R
, the ratios of R1, R2 and R3, define an average V
S
ADJ
(Refer to block diagram and Figure 1 - Operating waveforms)
IN
.
ADJ
is first applied, the initial current in L1 and R
SENSE
) appears directly at the (+) input of the comparator.
S
, L1 and the LED(s). The current rises at a rate determined by V
pin with respect to V
S
SENSE
) across R
5
IN
S
. The supply referred voltage V
). The average output current I
ADJ
ADJ
), the comparator output switches
, the comparator output switches
SENSE
S
S
), forms a self-oscillating
is zero and there is no
switching threshold of
ZXLD1352
www.diodes.com
www.zetex.com
IN
. The current
OUTnom
SENSE
IN
is
is